3 ARE WOUNDED IN JOHANNESBURG BLAST

By ALAN COWELL, Special to the New York Times

Published: January 4, 1987

JOHANNESBURG, Jan. 3—
An explosion apparently caused by a bomb went off today outside a 31-story office building in central Johannesburg. The police said three black women were wounded, one of them seriously.

The explosion, the first in the city center since a series of bomb explosions in June, went off in shrubbery against a wall at the offices of Sanlam, an insurance company run by businessmen from the nation's dominant Afrikaner minority.

It shattered windows in nearby apartments but appeared to have caused little major damage. The police cordoned off the area, barring reporters from inspecting the site. Television crews were barred from filming the scene.

No group immediately took responsibility for the explosion. In earlier instances, the authorities have blamed the African National Congress, the insurgent organization whose headquarters in exile are in Lusaka, Zambia.

A state of emergency was proclaimed in South Africa to combat black unrest that has claimed more than 2,300 lives since September 1984. The authorities sought to justify the newest tightening of the emergency decree, which outlaws virtually all forms of peaceful protest or dissent, by saying the African National Congress planned a Christmas terror campaign. The explosion today was the first reported since then. Rift With Cabinet Minister

In a separate development, President P. W. Botha was reported on state-run radio today to have delivered a sharp rebuff to a mixed-race Cabinet minister, the Rev. Allan Hendrickse, and to have renewed his commitment to the maintenance of racially segregated residential areas.

Mr. Hendrickse is one of two non-whites in the South African Cabinet and leads the segregated parliamentary chamber for people of mixed race in the nation's racially based, three-chamber Parliament.

Of the country's four main racial groups - whites, blacks, Asians and those of mixed race - blacks are not represented, and the mixed-race and Asian chambers, which were elected in a 1984 vote marked by widespread abstentions, lack authority to overrule the white, Afrikaner-dominated chamber.

Mr. Hendrickse told a convention of his Labor Party on Friday night that he might have to resign from Parliament if Mr. Botha refused to scrap legislation demarcating separate residential areas for the different racial groups.

Mr. Botha was quoted by state-run radio today, however, as saying that ''Government policy is to retain exclusive suburbs for the different population groups.''

Last year, the South African leader had hinted that an easing of the legislation, called the Group Areas Act, might have been on the agenda of his program of cautious racial change.

But, confronted with unabated violence and protest in segregated black townships, the program has all but stalled.

Additionally, Mr. Botha has announced elections for the white chamber of Parliament. Commentators say they expect him to avoid any action that might be interpreted by his constituents as weakness or appeasement.

The South Africa radio said Mr. Botha had denied that the Group Areas Act had been disputed in the Cabinet and termed his response to Mr. Hendrickse's threat a ''repudiation.''

The radio said Mr. Botha maintained that no system that deviated from the notion of ''group identity'' was acceptable. ''Group identity'' is a cornerstone of apartheid, reflecting the official contention that South Africa is a land of ethnic and racial minorities that should not be permitted to dominate one another.

Mr. Botha's statement seemed certain to erode the already frayed credibility of both the three-chamber Parliament and of Mr. Hendrickse, who joined the legislature saying he wanted to fight apartheid from within and would quit if this proved impossible.